The present invention relates to a camera tube apparatus for reading documents.
Heretofore, as a camera tube device for a documentreading apparatus for use in facsimile system and so forth, a MOS photo-diode array, a CCD device and the like have been used.
Since these camera tube devices are fabricated using integrated circuits (ICs), the size of the camera tube device per se can be miniaturized. In reading a document with such miniaturized camera tube devices, however, it is necessary to reduce the size of images by the use of an optical system using lenses or the like. As a result, a high accuracy is required in aligning the camera tube device with the optical system and difficulties have been encountered in miniaturizing the overall size of the manuscript reader.
In addition, a document reader has been proposed which uses camera tube devices having an array length equal to the width of the documents to be read. In the reader, 1:1 image formation is performed by an optical fiber array or a lens array used as the optical system. A light-receiving part of the camera tube device of the type described above has been fabricated by providing a photoconductive semiconductor on an insulating substrate, such as glass, for example, by using mask deposition techniques.
The use of the manuscript reader as described above permits the miniaturization of the manuscript reader because the image-formation path length can be reduced. In such an apparatus, however, it is difficult to construct a switching device for scanning and driving the camera tube device on the same substrate. Therefore, it has been necessary to interconnect the insulating substrate on which the light-receiving part is formed and the substrate on which the switching device is formed.
Hereinafter, the structure of a conventional camera tube apparatus for use in a document reader will be explained with reference to the accompanying drawings, wherein
FIG. 1 is a schematic view of the conventional camera tube apparatus as described hereinafter and
FIG. 2 is a sectional view taken along a line II--II.
Referring to FIG. 1, the conventional camera tube apparatus includes a camera tube device 1, a switching device 2, an insulating substrate 3, a split electrode 4, a photoconductive semiconductor layer 5 which is provided on the top surface of the insulating substrate 3 in such a manner that part of the layer is superposed on the split electrode 4, and a transparent electrode 6 which is provided on the top surface of the insulating substrate 3 in such a manner that part of the layer is superposed on the photoconductive semiconductor layer 5. Additionally, reference numerals 7 and 8 indicate, respectively, a switching IC and a connection electrode to connect the split electrode 4 and the switching IC 7.
In the foregoing conventional camera tube apparatus, the split electrode 4 is fabricated by providing a metal layer, such as gold, chromium or the like, in a thickness of about 500 to 1,000 .ANG. on the insulating substrate 3, which may, for instance, be glass, in such a manner that the pitch thereof corresponds to a desired degree of resolution, for example, in a ratio of 10 lines/mm.
The photoconductive semiconductor layer 5, which is constructed on the split electrode 4 with a thickness of 0.5 to 5 .mu.m by vapor-deposition, for example, is composed of amorphous or polycrystalline Se, Se-Te, CdS, CdSe or the like. The transparent electrode 6 is provided on the photoconductive semiconductor layer 5 in a thickness of 500 to 3,000 .ANG. by sputtering, for example.
The photoconductive semiconductor layer 5 is sandwiched between the split electrode 4 and the transparent electrode 6. The split electrode 4 and transparent electrode 6, which have an overlapped area corresponding to a picture element, form a light-receiving device P.
The switching device 2 is implemented with integrating switching ICs 7 (e.g., MOS-FETs) on the substrate. The switching device 2 includes connection electrodes 8, which extend to corresponding switches (not shown), arranged with the same pitch as the split electrodes 4. Each connection electrode 8 is connected to a corresponding split electrode 4 at the terminal thereof by a technique such as contact bonding.
FIG. 3 is an equivalent circuit diagram of the camera tube apparatus for explaining the driving system of the foregoing camera tube device 1. In the figure, the same symbols and reference numerals as used in FIG. 1 are used to indicate the same or equivalent parts. Cp1 to Cp5 indicate capacitances of the light-receiving devices P, D a photodiode formed by the light-receiving device P, Cl.sub.1 to Cl.sub.4, interline capacitances, S1 to S5 switches, Cs1 to Cs5, switch input capacities, t an output terminal, and R a load resistance.
Each connection electrode 8 is several centimeters long. If the wire density is high, significant interline capacitances Cl.sub.1 to Cl.sub.4 are present. In this case, among the light-receiving device capacitance Cp, interline capacitance Cl and switch input capacitance Cs, the relation of Cp&lt;Cl.ltoreq.Cs generally exists.
FIG. 4 is an equivalent circuit diagram of the camera tube apparatus used to explain the operation thereof with the output of the light-receiving devices P1 to P5 of the camera tube deivce 1 having the foregoing structure. In the figure, the same symbols and reference numerals as used in FIG. 3 indicate the same parts as in FIG. 3. Using a camera tube apparatus having the foregoing structure, a document is read as follows:
At the start of a reading operation, the switches S1 to S5 are closed to charge the capacitances Cp1 to Cp5 of the light-receiving devices P1 to P5 to a voltage of VO. Then, light-irradiation is applied onto the light-receiving devices P1 to P5 in a state where the switches S1 to S5 are opened.
Referring to the light-receiving device P1, electric charge is produced by the photodiode D according to the intensity of light irradiation. The charges thus generated flows into the corresponding switch capacitance Cs1 and is stored therein. Therefore, as time passes and charge flows, the potential at the point A increases. Then, when the switch S1 is closed, the switch capacitance Cs1 discharges, as a result of which the light-receiving device capacitance Sp1 is charged and at the same time the potential at the point A is decreased to the ground level.
At this time, by sensing a current i.sub.1 at an output terminal t, an output is obtained. In this case, since the potential at the point A changes, currents i.sub.2 to i.sub.5 flow as indicated in FIG. 4 through the interline capacitances Cl.sub.1 to Cl.sub.4 in addition to the signal current i.sub.1 from the light-receiving device P1. This leads to so-called "crosstalk" between adjacent channels or lines, and thus the output of the light-receiving device P1 is not as precisely representative of the document read as is desirable.